Prediction of Acute Mountain Sickness by Pulse Oximetry: What Are the Right Questions?

Abstract

Acute mountain sickness (AMS) is unpleasant. However, AMS is generally self-limited, easily treated, and largely preventable by gradual ascent. Portable pulse oximeters are simple to use and widely available, so it is tempting to try using them to predict who will get AMS. This might allay the anxieties of first-time travelers to high altitude and perhaps give those who are at increased risk more time to acclimatize and avoid getting AMS. As a recent editorial in Wilderness & Environmental Medicine1Windsor J.S. Pulse oximetry and predicting acute mountain sickness: are we asking the right questions?.Wilderness Environ Med. 2012; 23: 112-113PubMed Scopus (9) Google Scholar points out, prediction of susceptibility to AMS using pulse oximetry has been an elusive goal. The author concedes that 2 early studies using resting oxygen saturation by pulse oximetry (Spo2) showed high sensitivity but very low specificity to predict who will get AMS. There was no mention of the methodological flaws that call into question the reported sensitivity of 100% in each of the studies. The editorial referenced 2 new studies, also using resting Spo2, published in the same issue as the editorial, that were clearly negative.2Chen H.C. Lin W.L. Wu J.Y. et al.Change in oxygen saturation does not predict acute mountain sickness on Jade Mountain.Wilderness Environ Med. 2012; 23: 122-127PubMed Scopus (32) Google Scholar, 3Wagner D.R. Knott J.R. Fry J.P. Oximetry fails to predict acute mountain sickness or summit success during a rapid ascent to 5640 meters.Wilderness Environ Med. 2012; 23: 114-121Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar Karinen et al4Karinen H.M. Peltonen J.E. Kähönen M. Tikkanen H.O. Prediction of acute mountain sickness by monitoring arterial oxygen saturation during ascent.High Alt Med Biol. 2010; 11: 325-332Crossref PubMed Scopus (87) Google Scholar added measurements of Spo2 immediately after exercise to resting measurements and found statistically significant differences in the AMS and non-AMS groups. The actual results (their Table 3) show high sensitivity and low specificity similar to previous studies and their own use of resting Spo2. Attempts to use changes in Spo2 in individual subjects have also failed to yield a useful method of predicting who will get AMS.2Chen H.C. Lin W.L. Wu J.Y. et al.Change in oxygen saturation does not predict acute mountain sickness on Jade Mountain.Wilderness Environ Med. 2012; 23: 122-127PubMed Scopus (32) Google Scholar, 4Karinen H.M. Peltonen J.E. Kähönen M. Tikkanen H.O. Prediction of acute mountain sickness by monitoring arterial oxygen saturation during ascent.High Alt Med Biol. 2010; 11: 325-332Crossref PubMed Scopus (87) Google Scholar The editorial partly blames inaccuracies of pulse oximeters. Pulse oximeters used in the emergency department are generally accurate, but many factors such as carboxyhemoglobin and anemia can cause erroneous readings.5Lee W.W. Mayberry K. Crapo R. Jensen R.L. The accuracy of pulse oximetry in the emergency department.Am J Emerg Med. 2000; 18: 427-431Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar Some of the “[c]ommon sources of error” cited in the editorial, such as nail polish and acrylic decorations, are uncommon on treks and expeditions. Pulse oximeters may also be inherently inaccurate for technical reasons.6Milner Q.J. Mathews G.R. An assessment of the accuracy of pulse oximeters.Anaesthesia. 2012; 67: 396-401Crossref PubMed Scopus (46) Google Scholar It is well known that pulse oximeters become highly nonlinear and underestimate arterial oxygen saturations (Sao2) below 80%.7Luks A.M. Swenson E.R. Pulse oximetry at high altitude.High Alt Med Biol. 2011; 12: 109-119Crossref PubMed Scopus (63) Google Scholar The real problem with using pulse oximetry to predict AMS is the assumption that small differences in Spo2 correlate with the later development of AMS. The physiological basis for this is questionable. The Figure in the editorial shows large second-to-second fluctuations in Spo2 at high altitude that are larger than the supposed errors in pulse oximetry. I agree with Luks and Swenson,7Luks A.M. Swenson E.R. Pulse oximetry at high altitude.High Alt Med Biol. 2011; 12: 109-119Crossref PubMed Scopus (63) Google Scholar who concluded in their recent review of pulse oximetry at high altitude that “… clinical decisions should not be based on small differences in saturation over time or among individuals.” Two issues of the editorial should be noted and corrected. The author refers to the result of pulse oximetry as Sao2, which is arterial oxygen saturation measured using arterial blood gas analysis. The correct term is Spo2, which is arterial oxygen saturation measured by pulse oximetry. The source of the Figure in the article, showing Spo2 measurements in a resting subject at sea level and at high altitude, should be referenced and the Figure should be republished with the scales indicating the actual values on the x and y axes. One of the joys of practicing medicine in the wilderness is the ability to rely on clinical judgment without excessive testing. Pulse oximetry is an extremely useful clinical tool in managing patients with high altitude illness, but is unlikely to have value in predicting who will get AMS. Pulse Oximetry and Predicting Acute Mountain Sickness: Are We Asking the Right Questions?Wilderness & Environmental MedicineVol. 23Issue 2Preview“Will I get AMS?” is a question faced by many of those who look after groups in the mountains. After many years of being asked this question, I have developed a well-rehearsed answer. I begin first by giving an estimate of risk. For instance, if we are sitting in one of Lukla's teahouses on the evening of a trek to Mount Everest Base Camp, I will start by admitting that the condition could develop in as many as 30% during some stage of the trek.1 I will then go on to say that there will be some who by simple good fortune will have a lower risk. Full-Text PDF